Helical tomotherapy to LINAC plan conversion utilizing RayStation Fallback planning

Abstract RaySearch RayStation Fallback (FB) planning module can generate an equivalent backup radiotherapy treatment plan facilitating treatment on other linear accelerators. FB plans were generated from the RayStation FB module by simulating the original plan target and organ at risk (OAR) dose distribution and delivered in various backup linear accelerators. In this study, helical tomotherapy (HT) backup plans used in Varian TrueBeam linear accelerator were generated with the RayStation FB module. About 30 patients, 10 with lung cancer, 10 with head and neck (HN) cancer, and 10 with prostate cancer, who were treated with HT, were included in this study. Intensity‐modulated radiotherapy Fallback plans (FB‐IMRT) were generated for all patients, and three‐dimensional conformal radiotherapy Fallback plans (FB‐3D) were only generated for lung cancer patients. Dosimetric comparison study evaluated FB plans based on dose coverage to 95% of the PTV volume (R95), PTV mean dose (Dmean), Paddick's conformity index (CI), and dose homogeneity index (HI). The evaluation results showed that all IMRT plans were statistically comparable between HT and FB‐IMRT plans except that PTV HI was worse in prostate, and PTV R95 and HI were worse in HN multitarget plans for FB‐IMRT plans. For 3D lung cancer plans, only the PTV R95 was statistically comparable between HT and FB‐3D plans, PTV Dmean was higher, and CI and HI were worse compared to HT plans. The FB plans using a TrueBeam linear accelerator generally offer better OAR sparing compared to HT plans for all the patients. In this study, all cases of FB‐IMRT plans and 9/10 cases of FB‐3D plans were clinically acceptable without further modification and optimization once the FB plans were generated. However, the statistical differences between HT and FB‐IMRT/3D plans might not be of any clinically significant. One FB‐3D plan failed to simulate the original plan without further optimization.

Generally, treatment planning software system (TPS) is an integrated software package that allows the target and organs at risk (OAR) definitions, management of treatment plan, plan optimization, and delivery quality assurance (DQA). It also includes the DICOM import and export and data management system application software for archiving and management of patient data. TPS such as Eclipse (Varian Medical Systems, Palo Alto, CA, USA), Tomotherapy (Accuracy Inc, Sunnyvale, CA, USA), Pinnacle (Philips Healthcare, Andover, MA, USA), RayStation (RaySearch Medical Laboratories, Stockholm, Sweden) have different dose calculation engines as well as other characteristics that are unique to each system. Furthermore, each TPS needs to be commissioned using beam data from the linear accelerator to be used for patient treatment delivery. For example, a treatment plan generated from TPS that is commissioned to Varian Clinac iX linear accelerator could not be directly used to treat with Varian TrueBeam linear accelerator. In summary, there is no easy way to transfer patient treatment plans between different TPSs without repeating a significant amount of work.
Due to the lack of interchangeability among TPSs, there is a need to develop a method that can automatically transfer patient plans from one treatment unit/TPS to another treatment unit/TPS. This is especially useful for treatment centers that have multiple treatment units and TPSs that want to switch patients due to, for example, scheduling conflicts and machine down time.
Recently, RayStation TPS developed several advanced features to generate backup treatment plans. 1

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A FB plan was created by extracting information from a protocol plan generated using the FB module in RayStation TPS. The extracted information includes treatment planning parameters such as treatment techniques (3D, IMRT, or VMAT), beam geometry (gantry, collimator, couch, and other accessory settings), optimization parameters such as weighting factor of dose mimic between the target and the organ at risk (OARs). These parameters can be edited by the user and it is possible to test the FB protocol plan by using the dose mimicking technique to compare the FB plan and the original HT plan using a number of visual tools (i.e., dose volume histogram (DVH) curves, dose differences).
The precision of FB plan dose simulation is greatly related to the pregenerated protocol plan. The protocol plans can be used as a shared protocol plans such as tumor-specific protocol plans (lung, HN, prostate), treatment technique-specific protocol plans (IMRT, 3D, VMAT), energy-specific protocol plans (6 MV, 10 MV), beam angle-specific protocol plans (i.e., six field, seven field, or nine field), target position-specific protocol plans (i.e., head first or feet first).
The protocol plans also can be very specific used as a patient-specific protocol plan. A more specific protocol plan will result in a much higher degree of correspondence between the original HT plan and the resultant FB plan; however, a great deal of time and effort will be needed to generate these protocol plans.

2.C | Fallback plan creation
In this study, lung and HN IMRT FB plans shared the same single protocol plan for each patient with head first supine position and prostate IMRT FB plans shared another single protocol plan for each patient with feet first supine position. The protocol plan parameters used for all FB-IMRT plans included: nine field beams with fixed gantry angles of 40, 80, 120, 160, 200, 240, 280, 320, and 360 degrees; collimator angle of 0 degree; couch angle of 0 degree; and a static multileaf collimator (sMLC). The FB plan use dose mimicking optimization algorithm to optimize the Fallback plan. The goal of the dose mimicking optimization is to minimize the error in DVH between the reference plan (original HT plan) and the deliverable plan (Linac Fallback plan). Functions associated with OARs and targets are given a weighting factor equal to a user-defined target priority (Target/OARs ratios). In this study, the dose mimicking target/ OAR optimization weighting factor was set to 100.00 which means the importance of the optimization goal for target over OARs is 100.
Usually, the higher the ratio, the more importance for the target dose simulation and the lower the ratio, the more importance for the OARs dose simulation.
The energies of 6 MV were selected for lung and HN patients and 10 MV was selected for prostate patients. For FB-3D plans, patient-specific individual protocol plans were used. The plan parameters such as gantry, collimator, couch, and wedge angles for the FB-3D plans were determined individually and the final protocols selected were the ones that could best mimic the original HT plans.
For lung cancer patients, both FB-3D and FB-IMRT plans were evaluated. For HN and prostate cancer patients, only FB-IMRT plans were evaluated because IMRT treatment technique is the most commonly used treatment technique for HN and prostate cancer patients.
The quantitative evaluation of PTV dose distribution included: mean dose of PTV (D mean ), the PTV dose coverage R 95 where D x% is the dose to x% of the target volume, Paddick's conformity index (CI) 2 , and homogeneity index (HI). CI was defined by the following equation.
Where TV is the target volume, TV PIV is the target volume covered by the prescription isodose volume (PIV), and V PIV is the total prescription isodose volume. HI was defined by the following equa-

| RESULTS
The PTV dose coverage R 95 , D mean , CI, and HI from FB-IMRT and    We noticed that only the DVH of PTV-66 had an acceptable agreement between FB-IMRT and original HT plans comparing single-target dose simulation (Fig. 6) and multitarget dose simulation

ACKNOWLED GMENTS
The authors acknowledge the assistance from Hancock, Carolyn, CMD in proof-reading the manuscript and giving valuable feedback.
F I G . 6. Example of DVH calculated from FB-IMRT and HT plans with one PTV target (PTV66 and Target/OARs optimization weighting factor = 100) for one HN patient.